Re: [Xen-API] RRD library (was Re: Proposal to change committers for the XAPI Project)

[Jon Ludlam <jjl25@xxxxxxxxx>]

On 02/06/14 18:23, John Else wrote:
>
>> - Have you considered using vchan to let other tools capture the rrd stream 
>> more easily?
> I hadn't, but I think that could be made to work. At the moment the protocols 
> expect the shared-memory resource to be represented as a cstruct so it would 
> need a bit of tweaking to work with a socket-style interface instead.
>
> I also wanted to keep the library free of dependencies to any particular 
> handshake mechanism (e.g. xenstore), so I think the right interface would be 
> to start with a ready-initialised vchan server and turn that into a reader 
> (or conversely turn a vchan client into a writer).
A nice aspect of the current simple shared-memory design is that it
desynchronises reader and writer, so the writer can simply update its
page without worrying that the reader has died and isn't going to drain
the queue. It also means that the reader degrades nicely as the number
of data sources increases - it can simply sample less frequently. This
works best for DERIVE type data sources where the writer maintains a
monotonically increasing count of which the interesting thing is the
derivative (e.g. CPU time, or VIF bytes transmitted).

I think we should strive for the principle that observing the stats
should have as little impact as possible on the thing being observed, so
removing synchronisation points seems like a good idea. We clearly want
to avoid the Consciousness Causes Collapse problem :-)

Jon


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